Electromagnetic actuators using an electromagnetic attraction force are known in the prior art. FIGS. 20(a) through 20(c) show a prior-art electromagnetic attraction force generation mechanism constituting an electromagnetic actuator. FIG. 20(a) is a front view of the electromagnetic attraction force generation mechanism 101. The electromagnetic attraction force generation mechanism 101 is comprised of a magnetic body, such as iron, having a generally-rectangular cross-section. In particular, the electromagnetic attraction force generation mechanism 101 includes a pair of attracting iron cores 102a, 102b, extending in approximately the same direction, and a magnetic force generating iron core 103 connecting the ends of the attracting iron cores 102a, 102b, and thus has the shape of the letter “U”.
Wiring 104, composed of a linear conductive material such as a copper wire, is wound around the magnetic force generating iron core 103. The other ends of the attracting iron cores 102a, 102b are flat attracting surfaces 102as, 102bs. FIG. 20(b) shows the electromagnetic attraction force generation mechanism 101 of FIG. 20(a) as viewed in the direction of arrow A101, and FIG. 20(c) shows the electromagnetic attraction force generation mechanism 101 of FIG. 20(a) as viewed in the direction of arrow B101. The wiring 104 is omitted in FIGS. 20(b) and 20(c). As shown in FIGS. 20(b) and 20(c), the cross-sectional area of each of the attracting iron cores 102a, 102b is approximately the same as the cross-sectional area of the magnetic force generating iron core 103.
FIG. 21 shows an electromagnetic actuator 111 using the electromagnetic attraction force generation mechanism 101. In the electromagnetic actuator 111, the attracting surfaces 102as, 102bs of the electromagnetic attraction force generation mechanism 101 are held approximately vertical by means of a not-shown holding mechanism. A movable iron piece 106 is disposed in a position opposite the attracting surfaces 102as, 102bs of the electromagnetic attraction force generation mechanism 101 with a slight gap 105 between them, as shown by the solid lines. The length of the gap 105 between one surface 106s1 of the movable iron piece 106 in that position and the attracting surfaces 102as, 102bs is x101.
The opposite surface 106s2 of the movable iron piece 106 is connected via a wire 107a to one end of a spring 108, and the other end of the spring 108 is connected via a wire 107b to a wall surface 109. The surfaces 106s1, 106s2 of the movable iron piece 106 are approximately vertical; the attracting surfaces 102as, 102bs of the electromagnetic attraction force generation mechanism 101 are approximately parallel to the opposing surface 106s1 of the movable iron piece 106.
The operation of the electromagnetic actuator 111 will now be described with reference to FIG. 21. When a voltage is applied to the wiring 104, an electric current is supplied to the siring 104 and a magnetic flux is generated and increased in the flowing magnetic circuit: magnetic force generating iron core 103→attracting iron core 102a→gap 105→movable iron piece 106→gap 105→attracting iron core 102b→magnetic force generating iron core 103. Accordingly, an attraction force is generated and is applied from the attracting surfaces 102as, 102bs to the surface 106s1 of the movable iron piece 106 via the gap 105. Therefore, the spring 108 extends and the movable iron piece 106 is displaced toward the attracting surfaces 102as, 102bs, and the surface 106s1 is attracted and attached to the attracting surfaces 102as, 102bs, as shown by the broken lines in FIG. 21. Thus, the length of the gap 105 becomes substantially zero.
The movable iron piece 106 moves while maintaining the approximately vertical position by means of a guide or a parallel spring as a guide, both not shown. The surface 106s1 of the movable iron piece 106 can therefore be kept parallel to the attracting surfaces 102as, 102bs of the electromagnetic attraction force generation mechanism 101 during the movement of the movable iron piece 106.
When the voltage applied to the wiring 104 is shut off, the electric current disappears, whereby the magnetic flux in the magnetic circuit decreases. Due to the biasing force of the spring 108, the surface 106s1 of the movable iron piece 106 moves away from the attracting surfaces 102as, 102bs and returns to the position shown by the solid lines in FIG. 21, i.e. the position where the length of the gap 105 between the surface 106s1 and the attracting surfaces 102as, 102bs is x101. Thus, the displacement produced in the movable iron piece 106 by means of the electromagnetic attraction force generation mechanism 101 is x101.
Such electromagnetic actuator 111 has the following problems: FIG. 22 is a graph showing the relationship between displacement and thrust force in the electromagnetic actuator 111, as observed when a constant electric current is supplied to the wiring 104. In FIG. 22, the abscissa represents the displacement x101, and the ordinate represents the attraction force, i.e. the thrust force, applied from the electromagnetic attraction force generation mechanism 101 to the movable iron piece 106 when the displacement is produced. As can be seen in FIG. 22, though the thrust force is sufficiently high when the displacement is small, the thrust force drastically decreases as the displacement increases.
Thus, the attraction force, i.e. the thrust force, applied from the electromagnetic attraction force generation mechanism 101 to the movable iron piece 106 is significantly low when the length of the gap 105 (displacement) x101, shown in FIG. 21, is large as compared to the case where the displacement x101 is small; the thrust force applied to the movable iron piece 106 is very low when the movable iron piece 106 lies in a position farthest from the attracting surfaces 102as, 102bs of the electromagnetic attraction force generation mechanism 101.
When it is intended to produce some effect, e.g. the generation of vibration, by using the thrust force, only a very low vibration force can be obtained when the thrust force is very low. Thus, in order to obtain a sufficiently high thrust force in the prior-art electromagnetic actuator 111, the displacement must be limited to a very small value range. To obtain a sufficiently high thrust force with the use of a large displacement, it is necessary to supply a high electric current to the wiring 104 of the electromagnetic attraction force generation mechanism 101. This requires the use an electronic part(s), which is adapted for high electric current, in a current supply circuit for the wiring 104, leading to an increase in the cost or size of the circuit. In addition, because of non-integration of the electromagnetic actuator 111 as a whole, parts such as the electromagnetic attraction force generation mechanism 101, the movable iron piece 106, the wires 107a, 107b and the spring 108 are produced separately and thereafter assembled. This requires a complicated process for the production of the electromagnetic actuator 111.